Dear ,ory
Depending on fermentation activity, the colonies get very different hues
of red on MacConkey. Very strongly acid-producing strains will give
precipitations of bile acids in the agar surrounding the colonies in
addition to being bright red. The amount of acid produced will strongly
depend on the type of sugars used, the metabolic pathways for their
degradation, and the type of fermentation end products observed. Finally,
weak acid producers may revert the color of the agar because the alcaline
byproducts of the aerobic metabolism of the peptone present in the medium
will not only neutralize the acid produced in the beginning, but change
the medium pH sufficiently to make the neutral red indicator in the
medium turn yellow.
Hope this is of use to you, Andre
--
Andre P. Burnens MD
Institute for Veterinary Bacteriology, University of Berne
Laenggass-Strasse 122, CH-3012 Berne, Switzerland
Tel. +41 31 631 24 85 Fax. +41 31 631 26 34
